The conventional method of molding a case for a push button switch is described below by referring to FIGS. 6-9. Referring first to FIG. 6, there is shown a push-button switch of this kind. This switch has a case 1 made from synthetic resin. A central fixed contact 2 is inserted in the bottom wall of the case 1. Also, fixed contacts 3 are inserted in the bottom wall and disposed on opposite sides. Terminals 4 extend from the fixed contacts 2 and 3 and protrude downwardly from both sides of the case. A curved movable contact 5 is disposed on the fixed contacts 3. A stem 6 having a protrusion 6a is disposed on the movable contact 5 to permit the operator to depress the movable contact. An annular member 7 is mounted on the case 1 to limit upward movement of the stem 6.
The manner in which the case 1 of this pushbutton switch is molded is described now by referring to FIG. 7. A hoop-like contact plate 8 having the central fixed contact 2 and the opposite fixed contacts 3 is disposed between an upper fixed die 11 and a lower movable die 12. After the dies 11 and 12 are tightened together, resin such as polyethylene terephthalate or polybutylene terephthalate is molten within a molding machine 13. The resin is injected into a cavity 17 formed between the dies 11 and 12 from the molding machine 13 through a spray 14, a runner 15, and a submarine gate 16 that is formed in the lower movable die 12 and extends to one side of the cavity. In this way, the case 1 is molded out of the resin.
Where the case 1 is molded in the manner described above, the fixed contacts 2 and 3 are placed only on support portions 12b and 12c on a protruding block 12a of the lower movable die 12. Therefore, a gap tends to be produced between the fixed contact 2 or 3 and the support portion 12b or 12c. If resin intrudes into this gap, the resin covers the fixed contacts 2 and 3. This introduces the problem that when the switch is used, a perfect contact will not be made between the movable and fixed contacts.
Accordingly, as shown in FIG. 8, a locating pin 19 whose front end is curved protrudes from the upper movable die 11 and bears against the central fixed contact 2. Locating pins 20 whose front ends are flat protrude also from the upper die 11 and bear against the opposite fixed contacts 3. In this way, the fixed contacts 2 and 3 are pressed against the lower die 12 to prevent the fixed contacts 2 and 3 from being covered with the resin when the case 1 is molded.
The conventional method of molding the case makes use of the locating pins 19 and 20 which protrude from the upper fixed die 11, in order to prevent the formation of a gap between the fixed contact 2 or 3 and the lower die 12. The pins 19 and 20 place the fixed contacts 2 and 3 in position. Since these pins 19 and 20 are cylindrical in shape, after the case 1 is molded on the contact plate 8, holes 1a and 1b are formed between the bottom surface of the case 1 and the fixed contacts 2 and 3, as shown in FIG. 9, to allow the pins to be withdrawn.
When the terminal 4 of the push-button switch is soldered to a printed-wiring board 21, if the soldering operation is performed manually, then almost no problem will occur. However, if the terminal 4 is automatically dipped in solder, then soldering flux which is sprayed from the underside intrudes into the holes 1a in the case 1 via terminals 21a of the board 21, the holes 1a being formed to withdraw the locating pins 20 for supporting the opposite fixed contacts 3. Sometimes, the flux intrudes into the hole 1b in the case 1, the hole 1b being formed to withdraw the locating pin 19 that holds the central fixed contact 2. Then, the flux moves upward through a narrow gap between the case 1 and the fixed contact 2 or 3, so that it adheres to these contacts 2 and 3. Consequently, the electrical connection of the movable contact 5 with the fixed contacts 2 and 3 deteriorates, and a good contact between them is not made.